The Museum of Modern Art [Apple] curates its collection so that people get what they expect when they visit the museum. That is, they see good quality modern art. If you want to see other types of art, it doesn't do much good to whine about the rules. Just go to any of the other museums where you can find all kinds of art.
Your museum analogy fails because you didn't tie it to a device. With my iPhone, I have no choice to go to Apple for apps.
Try another analogy or rework it. Of course, then your points falls apart...
Apple did good here. They can have a curated store without having unjustified restrictions that limit developer and consumer options without any advantage.
I can write malware in Objective-C using the Apple frameworks. Having 3rd party frameworks and allowing different programming language does not open the door to anything that can't be done right now.
Apple took the right decision here, after making such a poor one in April in an attempt to block a competitor unreasonably.